The amount of radioactive iodine-131 isotope in the samples, taken Wednesday some 330 meters (361 yards) into the Pacific Ocean, has surged to 4,385 times above the regulatory limit. This tops the previous day's reading of 3,355 times above the standard -- and an exponential spike over the 104-times increase measured just last Friday.

Officials have downplayed the potential perils posed by this isotope, since it loses half of its radiation every eight days.

Yet amounts of the cesium-137 isotope -- which, by comparison, has a 30-year "half life" -- have also soared, with a Wednesday afternoon sample showing levels 527 times the standard.

"That's the one I am worried about," said Michael Friedlander, a U.S.-based nuclear engineer, explaining cesium might linger much longer in the ecosystem. "Plankton absorbs the cesium, the fish eat the plankton, the bigger fish eat smaller fish -- so every step you go up the food chain, the concentration of cesium gets higher."

On Thursday, Hidehiko Nishiyama, a Japanese nuclear safety official, reiterated that seawater radiation doesn't yet pose a health risk to humans eating seafood.

Fishing is not allowed within 20 kilometers (12 miles) of the plant, and he claimed that waterborne radiation should dilute over time.

Still, authorities don't know where the highly radioactive water is coming from.